Frits which are used to form enamel coatings on underlying surfaces of iron or steel usually have fusing temperatures which are above about 1460.degree. F. Those frits generally cannot be used on aluminum, which has a melting temperature of about 1217.degree. F., well below that of such frits. (The common aluminum alloys melt at temperatures that are slightly higher than unalloyed aluminum but still substantially below the melting point of such frits.) In order to be fusible on a surface utensil of aluminum or its alloys, a frit must, among other criteria, have a fusing temperature no higher than about 1050.degree. F. In order to formulate such a frit, lead oxide was once included as a component of the frit; it acted as a flux and lowered the fusing temperature. However, it is now well known that even very small quantities of lead (or lead oxide) in a frit can be toxic, where the frit is on a utensil which comes into contact with food. In use of the utensil the lead oxide will gradually be dissolved from the enamel and may appear in food or liquid heated or served in the utensil. It therefore has become desirable to avoid the use of all lead oxide in frits to be applied to eating or cooling utensils.
One substitute for lead oxide, as a means of reducing the fusing point of a frit, was bismuth oxide. Bismuth is not itself toxic; however, it was found that virtually all large volume, low cost, sources of bismuth oxide contained small proportions of lead oxide as a trace impurity which was carried over into the frit. Even if lead oxide is only present in an amount greater than 5 ppm, it is potentially harmful. Thus it has not been practical to provide a frit suitable for use on aluminum by using bismuth oxide; effective amounts of it tend to carry more than about 5 ppm lead into the enamel.
The elimination of lead and bismuth oxides as fluxes for producing low melting frits has made it difficult to discover a frit which has an adequate overall combination of the necessary properties. In addition to having a fusing temperature below about 1050.degree. F., a frit for use on aluminum must also have adequate chemical durabililty, which is generally characterized as acid resistance. Further, it must have good adherence to the underlying base, whether a casting, extrusion or sheet aluminum; it must have good spall resistance; it must be capable of accepting coloring agents and pigments to yield a range of opaque colors from very dark to pastels; and after firing it must display a reflectance and a specular gloss which are adequate to meet aesthetic requirements.
It has proven unusually difficult to discover a frit which meets these requirements while at the same time avoiding the use of essentially all lead and other toxic heavy metal oxides.